HIV infection begins when the virus fuses with target cell membranes. The envelope glycoprotein (Env) of HIV mediates the fusion process by undergoing complex conformation changes while it binds to target cells, The lack of fundamental knowledge about HIV Env structure impedes efforts to rationally design vaccines to block HIV infection and may also account for our lack of understanding of why current vaccines are failing. Our research aims to elucidate the molecular details of Env structure and function during membrane fusion. Our research projects focus on alpha-helical segments in the transmembrane protein (TM) of Env, because analogous segments in other viruses suggest that these highly conserved structural domains play important roles in viral enty. In addition, peptides corresponding to these alpha-helical segments potently inhabit HIV entry and fusion, and studies of the mechanism of inhabition by these peptides should provide valuable insights in the fusion mechanism of Env. We have several studies underway including: 1) characterization of inhibition by peptides; 2) site-directed mutagenesis of alpha-helical domains to determine effects on fusion activity and inhabition by peptide; 3) generation and characterization of escape mutant viruses resistant to inhibition by peptide; 4) studies of Env chimeric in alpha-helical regions in TM. During this past year we have completed characterization of one peptide, and are almost done characterizing inhibition of a second peptide. We have also made and partially characterized a panel of over 30 mutant Envs.